1. Field of the Invention
The present invention relates generally to artificial speech aids, and more particularly, to a method and apparatus for generating non-audible artificial speech.
2. Discussion of the Prior Art
A number of devices have been developed which either replace the larynx (including the vocal cords) or otherwise artificially create comprehensible speech. The production of speech begins in the lungs which supply the air that is required to vibrate the vocal cords and carry the tune that is produced through the throat or pharynx and out of the nose and mouth. When a person desires to produce speech, the muscles controlling the vocal cords are tightened, thereby allowing the air passing through the larynx to cause the vocal cords to vibrate, which in turn produces a tone. The amplitude of the tone produced by the vocal cords is proportional to the amount of air supplied by the lungs. Modulation of the tone into comprehensible speech is caused by variations in the movement and positioning of the tongue, nasal cavity, throat and mouth.
Prior art artificial speech generation devices have been primarily designed to replace the larynx or vocal cords of persons whom have suffered either a permanent or temporary injury to that portion of their vocal track. Due to the injury suffered, the person is incapable of producing a tone for modulation. To solve this problem, U.S. Pat. No. 4,571,739, issued Feb. 18, 1986, to Resnick, discloses using an artificial tooth inserted into the mouth, constructed out of tone emitting components, for producing a tone which may then be modulated by the mouth, pharynx and nasal cavity of a laryngectomy patient. Similarly, U.S. Pat. No. 4,550,427, issued Oct. 29, 1985, to Katz et al, disclose a completely self-contained intraoral artificial larynx, having its own power source, tongue activated control, power saving signal generation circuitry, acoustic and audio amplifiers, and intraoral speaker, all of which are contained within an otherwise conventional dental prosthesis. Like the Resnick device, the device in Katz et al is designed for usage by laryngectomy patients, and in addition to tone production, provides some tone amplification and enhancement. Both prior art devices are designed to be operated only when the larynx is inoperative or missing.
Additional prior art devices also supply a certain quantity of air, in addition to the tone, into the oral cavity. In U.S. Pat. No. 4,612,664, issued Sept. 16, 1986, to Walsh et al, a solenoid reed valve capable of producing an amplitude modulated tone is disclosed. The reed valve replaces the amplitude modulation function of the lungs by regulating the quantity of air passing through a pressurized air passage created in the valve, which in turn creates a tone of sufficient amplification. U.S. Pat. No. 4,627,095, issued Dec. 2, 1986, to Thompson, discloses an artificial voice apparatus having a pump for producing an air flow within a tube connected to a sounding mechanism, thereby combining an audible tone with air flow. The tube is then coupled to a mouth piece and tone is transported into the person's mouth where the tongue, lips and teeth articulate the sound into speech. In these prior art devices, it is assumed that an audible tone is required to replace the vocal cords of the person using the device. However, if a person simply desires to speak without using their vocal cords, the transmission of an audible tone into the person's mouth would make it impossible to create artificial speech while at the same time restricting the audible emission of that speech from the mouth.
A large number of applications exist for nonaudible speech communication, such as in surveillance operations, military operations, or even where a person simply wishes to not be overheard while speaking in a private telephone conversation. Likewise, there are a number of situations in which surrounding or background noise is so great, that common levels of conversation or even high amplitude speech cannot be heard, such as at airports, on the battlefield, or in industrial environments. Finally, there are a number of applications, where audible speech itself, although desired, may be too awkward or distracting, such as in the case of dictation where the dictator is apparently only talking to himself. In such an instance, silently communicating to the dictation unit, which can record and playback an audible recording, may be an easier and more convenient means of communication than audible speech, and just as effective. However, inaudible artificial speech applications have previously been made unachievable with prior art artificial speech generation devices.